This invention relates to brazed aluminum products, and particularly to the behavior of such products when exposed to corrosive environments.
Brazing is a widely used process for joining aluminum parts in the manufacture of such products as air conditioning evaporators and radiators. In use, these products, particularly automotive radiators, are exposed to salted road splash causing intergranular corrosion attack, which limits the useful life of the product.
This problem has been addressed in the literature in a variety of ways. An early example is Miller, U.S. Pat. No. 2,821,014 (Jan. 28, 1958), where it is disclosed that intergranular corrosion problems in flux and dip brazing are alleviated by adding an interlayer between the structural member portion and the brazing layer. The interlayer is aluminum or an aluminum-base alloy, particularly certain magnesium-containing alloys, having a melting point greater than that of the structural alloy. The solution offered by Singleton et al., U.S. Pat. No. 3,788,824 (Jan. 29, 1974) and its divisional, U.S. Pat. No. 3,881,879 (May 6, 1975), is directed to vacuum brazing, and involves the addition of iron to either the core alloy or the cladding alloy as an alloying element, resulting in improvements in both corrosion resistance and sag resistance. Anthony et al., U.S. Pat. No. 4,039,298 (Aug. 2, 1977) address both flux and vacuum brazing, and disclose a composite of complex and highly specified composition as being particularly beneficial in terms of corrosion properties. The disclosed core alloy contains specified amounts of manganese, copper, chromium, silicon and iron as alloying elements with both a solid solution and an alpha-phase, whereas the alloying elements in the cladding are bismuth and silicon. An additional disclosure by the same patentees appears in U.S. Pat. No. 4,093,782 (June 6, 1978) and its continuation-in-part, U.S. Pat. No. 4,167,410 (Sept. 11, 1979), in which the core alloy contains a specified combination of chromium and manganese, with resultant improvements in both corrosion resistance and sag resistance. A similar disclosure appears in Setzer et al., U.S. Pat. No. 3,994,695 (Nov. 30, 1976), where the core alloy contains a chromium-manganese-zirconium combination, the sole claimed benefit however being an improvement in sag resistance. A combination of copper and titanium as primary alloying elements in the core alloy is disclosed in Kaifu et al., U.S. Pat. No. 4,339,510 (July 13, 1982), as providing a benefit in intergranular corrosion resistance.
A different approach is disclosed by Nakamura, U.S. Pat. No. 4,172,548 (Oct. 30, 1979), in which corrosion following fluxless brazing processes in general (including both vacuum brazing and brazing in an inert atmosphere) is controlled by controlling the grain size of the brazing sheet to at least 60 microns in diameter, achieved by a controlled cold work followed by a full anneal.
Heat treatment is disclosed for metallic alloys in general for a number of reasons. Heat treatment after brazing in a heat-hardenable copper-based alloy is disclosed by Silliman, U.S. Pat. No. 2,117,106 (May 10, 1938) for returning hardness and spring qualities lost during the brazing procedure. Soldered joints in copper-brass radiators are heat-treated in a process disclosed by Harvey, U.S. Pat. No. 3,335,284 (Nov. 28, 1967) to lessen the occurrence of stress and creep-rupture at the operating temperature. Tisinai, et al., U.S. Pat. No. 3,028,268 (Apr. 3, 1962) used a high temperature heat treatment to impart corrosion resistance to nickel-chromium-molybdenum alloys. Heat treatments are disclosed for similar purposes in aluminum wires (Weber, U.S. Pat. No. 3,503,596, Mar. 31, 1970) and zinc-based alloys (Gervais, et al., U.S. Pat. No. 3,880,679, Apr. 29, 1975).